. . . they always picture holograms as free-standing, 3-D projections? (Frequently blurry, with lines running through them, like on a badly adjusted, low-def TV set? I'm thinking of you, George Lucas.)
Actually, real-world holograms tend to be flat (e.g., printed on a flat surface, like your credit card), but you see the 3-D effect if you stare into them.
The other kind, the kind you see in sci-fi, is known as a volumetric display.
We only have the beginnings of that technology now.
In sci-fi, volumetric displays are often portrayed as insubstantial projections, presumably of laser light.
But there are other alternatives. . .
